1. Technical Field of the Invention
The invention relates generally to CRT (cathode ray tube) video monitors in computer systems, and in particular, to a multiplexed video interface system.
2. Description of the Related Art
A computer system essentially comprises a system unit housing a microprocessor, computer memory, and various other support logic, as well as various input/output (I/O) devices which are connected to the system unit and enable a user to intelligently interact with the system unit. Examples of various types of input devices include a keyboard, a mouse, a trackball, and a pen computer, as well as others. The primary output device in a computer system include a video display monitor (video monitor).
Video monitors, such as for use with digital computers, include a cathode ray tube (CRT), and driver circuitry including a video amplifier. The CRT includes three primary color cathode ray guns which are manipulated to converge on a screen that produces the color image. The three guns produce converged scanning rasters having red, green and blue fields which combine to produce white light. The typical scanning raster is a left to right horizontal and top to bottom vertical scan operated in accordance with the Video Electronics Standards Association (VESA) requirements.
A conventional monitor amplifier circuit 100 for displaying screen control states of a monitor is illustrated in FIG. 1. In general, low level color video signals blue b, red r, and green g from a video source (not shown), such as a personal computer (PC) are provided to respective video preamplifiers 101a, 101b and 101c. These preamplifiers in turn provide the respective video signals blue b, red r, and green g, via buffer amplifiers BUFF11, BUFF12, BUFF13, to video output driver stages 103a, 103b, 103c which supply high level amplified color video signals B, R and G to respective cathode intensity control electrodes of a CRT (not shown). As can be seen, in FIG. 1, each video signal blue b, red r, and green g is applied to a respective amplifier circuit AMP11-AMP13, each of which includes four main components: a video preamplifier 10la-101c, a bias/brightness circuit 105a-105c, a video amplifier 103a-103c, and a clamp amplifier 107a-107c. Since the monitor amplifier circuits AMP11-AMP13 are identical in structure and operation, only the circuit operation of amplifier circuit AMP12 for the red video signal r will be discussed by referring to FIG. 2.
As can be seen in FIG. 2, the four main components of monitor amplifier circuit AMP12 are numbered 1-4, number 1 being bias/brightness circuit 105b, number 2 being video preamplifier 101b, number 3 being clamp amplifier 107b, and number 4 being video amplifier 103b.
Operation of this red video channel r is as follows. Terminal 10 constitutes the red video signal input r which originates from an external source, such as a PC. Capacitor CAP12 couples the red video signal r to the non-inverting input of video preamplifier 101b.
At this point, the amplification of red video signal r is controlled by a single-throw switch SW12 and a video clamp pulse VC. In any video signal, the clamp pulses are developed just following the synchronization pulses and make it possible to restore the voltage reference level of a video signal, in this case red video signal r. This clamp pulse VC is located in the "back porch" of the composite red video signal r and is employed to operate switch SW12. When clamp pulse VC is high, switch SW12 is closed. Thus, each time the CRT scans a horizontal line, capacitor CAP12 will be charged to black level reference voltage VREF, which is the potential reference level of the black region of an image. This level corresponding to the black color in an image makes it possible to restore the potential reference level of the red video signal r, this level having disappeared on account of the presence of the input capacitor CAP12.
On the other hand, when video clamp pulse VC is low, switch SW12 opens and red video signal r is applied directly to video preamplifier 101b, which is shown in FIG. 2 as a unity gain amplifier. Thus, red video signal r is passed through video preamplifier l01b.
At this point, the amplification of red video signal r is controlled by double-throw switch SW14 and signal 11. Signal 11 represents a horizontal blanking pulse that is derived from the display scanning circuits in a manner well known in video display monitors. This signal 11 is employed to operate a double-throw switch SW14 which switches the input IN12 to output buffer BUFF12, between the output of video preamplifier 101b and circuit ground. When signal 11 is high, input IN12 couples to video preamplifier 101b, the output of which is inversely amplified by video amplifier 103b to a voltage level suitable for driving a CRT and then applied to cathode electrodes of the CRT. On the other hand, when signal 11 is low, input IN12 is at circuit ground and the CRT is blanked by driving the output of the video amplifier 103b to a high level.
During operation of this amplifier circuit AMP12, output coupling capacitor CAP 22 changes the DC level at the CRT cathode. Thus, a bias clamp circuit 105b is used to restore the DC level at the CRT cathode through a series diode D11. Bias clamp circuit 105b outputs a bias clamp DC voltage which, in a typical video monitor, is usually factory set. This bias clamp voltage reinstates the charge on output capacitor CAP22 only during the blanking period. The voltage is preset, typically, in the range of 100-140 volts to compensate for differences in CRT cathode bias levels, required by each cathode in the CRT to set the black level. In addition, an adjustable voltage component of typically +/-10 volts may be added to this bias level to accomplish the `brightness` feature, such that the black level can be manually adjusted by an external source. Thus, for example, increased image brightness results when the bias clamp voltage is reduced. This results in a less positive DC bias potential at the red cathode and a related increase in image brightness.
Although the conventional monitor amplifier system 100 amplifies and conditions video signals to drive the CRT, there are several disadvantages to the circuit configuration. Referring again to FIG. 1, it can be seen that this architecture involves a significant number of interconnections. Such a low level of integration has several disadvantages. First, the circuit architecture requires a large printed circuit board (PCB), yielding higher design costs due to shielding for the radio frequency (RF) interface. Second, the conventional circuit architecture has inferior high frequency performance due to long interconnection traces between the components and due to electromagnetic interference (EMI) stemming from long signal lines and large signal swings across the video interface between each preamplifier 101a-101c and corresponding video amplifier 103a-103c. Third, the high number of interconnections require higher pin count packages which are undesirably large and expensive. Finally, the complexity of the system 100 due to the low level of integration results in longer design time.
Thus, a need exists for a monitor amplifier system with a simplified architecture having a high level of integration.